A damper operates in vehicle suspensions as a damping device controlling the sprung (body) and unsprung (wheels) masses of a vehicle by reducing loads or vertical accelerations normally transmitted from the wheels to the body. Damping is accomplished by converting kinetic energy into thermal energy and dissipating the heat. Conventionally, hydraulic dampers include a piston with a connected piston rod slidably carried in a fluid-filled tube and separating the tube into extension and compression chambers. A rod guide at the top end of the tube closes the extension chamber and slidably engages the piston rod. In many applications, the suspension damper is called upon to limit the full extension travel of the suspension system.
In some vehicle applications, mechanical rebound stops that are fixed to the piston rod and engagable with the rod guide are known to provide a means of limiting the maximum extension travel of the piston rod from the damper. A typical mechanical rebound stop is generally equipped with a resilient bumper made of material such as rubber or urethane. The bumper is designed to cushion the engagement of the damping piston with the rod guide at the end of damper travel in the extension direction. This type of a mechanical stop tends to result in somewhat of an abrupt means of limiting travel during rebound. It has also been found that in severe applications, a resilient bumper material may undesirably experience heat degradation when the bumper absorbs the entire rebound stop load.
Providing a hydraulic rebound cut-off feature with a shock absorber form of damper is also known. Such a device is disclosed in U.S. Pat. No. 3,621,949 which issued Nov. 23, 1971. That hydraulic rebound cut-off feature uses an axial rod flow path communicating fluid between the extension and compression chambers and operates as an additional damping flow path with the typical piston valve orifices. When the piston approaches full extension, the axial rod flow path is closed by the rod guide, significantly increasing damping force and slowing extension directed travel. This prior art device undesirably necessitates changes to the rod guide to prevent blow-by of hydraulic fluid caused by high pressure which builds in the lower entrance segment of the rod guide and requires a larger rod I.D. for sufficient flow capacity.
Yet another type of known hydraulic rebound cut-off feature utilizes a rebound cut-off piston in conjunction with the damping piston. Such a device is described in U.S. Pat. No. 3,007,550 which issued Nov. 7, 1961. According to this prior art design, a fixed/clamped disc or disc stack on a secondary or rebound cut-off piston coacts with the damping piston to effect a substantial entrapment of fluid in the extension chamber of the shock absorber as the damping piston approaches full rebound. However, this device effects an increase in dead length and resistance to movement only when the velocity of movement of the damping piston exceeds a certain predetermined value. In so doing, a rather complicated assembly is used that undesirably detracts from active length of the damper.
Particularly with monotube design dampers, maximizing active length is critical. This is because a typical monotube damper carries a gas cup that separates out a gas chamber within the single tube of the device. The gas chamber is expansible and contractible to account for the changing volume of space occupied by the piston rod entering and exiting the tube and accommodates thermal expansion of the fluid. Presence of the gas chamber minimizes the amount of active length that can be utilized by other features such as the rebound cut-off device. A piston rod flow design is not possible for the rebound cut-off device because the hollow rod is used for the electrode assembly and has an insufficient diameter. Also, a fixed/clamped disc would increase disc to sleeve clearance, thereby reducing the rebound cutoff effect. Therefore, there is a need in the art for a self-aligning rebound cut-off feature for a monotube damper with minimal impact on damper dead length.